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Dec 10, 2014 - University of Belgrade, Vinča Institute of Nuclear Sciences, P.O. Box 522, ... Institute of Physics, University of Tartu, Tartu 50411,...
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Deep-Red Emitting Mn4+ Doped Mg2TiO4 Nanoparticles

Mina M. Medić,† Mikhail G. Brik,‡,§,∥ Goran Dražić,⊥ Ž eljka M. Antić,† Vesna M. Lojpur,† and Miroslav D. Dramićanin*,† †

University of Belgrade, Vinča Institute of Nuclear Sciences, P.O. Box 522, Belgrade 11001, Serbia College of Sciences, Chongqing University of Posts and Telecommunications, Chongqing 400065, People’s Republic of China § Institute of Physics, University of Tartu, Tartu 50411, Estonia ∥ Institute of Physics, Jan Dlugosz University, PL-42200 Czestochowa, Poland ⊥ Laboratory for Materials Chemistry, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia ‡

ABSTRACT: Synthesis, structure, morphology, and detailed spectroscopic and crystal-field analysis of Mn4+ doped Mg2TiO4 nanoparticles (NPs) are presented. These Mg2TiO4:Mn4+ NPs are obtained through a Pechini-type polymerized complex route and calcination at 600 °C, and are approximately 10 nm in diameter and loosely agglomerated into 1-μm particles, as evidenced from transmission electron microscopy. These NPs exhibit strong, sharp red emission at 658 nm (with a 1.2 ms emission decay) as a result of the spin-forbidden 2Eg → 4A2g electron transition of the tetravalent manganese ions. No signatures of the presence of manganese ions in divalent or trivalent valence states are observed in the NPs with either photoluminescence or diffuse reflection spectroscopy. The energy levels of the Mn4+ ions in a trigonal crystal field of Mg2TiO4 are calculated using the exchange-charge model and are well matched with the experimental photoluminescence excitation and emission spectra. The absolute values of the calculated crystal-field parameters (CFPs) are similar to those reported for trigonal point symmetry at Mn4+ dopant sites (Y2Ti2O7, Y2Sn2O7, and Na2SiF6). It is also observed that the contributions of covalent and exchange effects to the CFPs are nearly eight times greater than the point-charge contribution. → 4A2g transition of the manganese ions. The energy of the 2Eg excited state does not depend on the crystal-field strength.11 However, depending on the host, the spectral position of the 2 Eg → 4A2g transition maxima can be detected over a wide range: from 620 nm in K2SiF6 to 723 nm in SrTiO3.12 As previously reported,12 the covalent interaction between Mn ions and their surrounding environment is responsible for such varied emission wavelengths. Among the various materials that can be doped with Mn4+ ions, crystals with the spinel structure are a particularly noteworthy example. To begin with, they are chemically stable. Second, they have a cubic structure, and the Mn4+ site is characterized by octahedral or trigonal symmetry, which makes crystal-field analysis and comparison with the Tanabe−Sugano diagram more reliable without needing to consider lowsymmetry crystal-field effects. Examples of earlier studies of Mn-doped spinel materials can be given as follows: Mn-related luminescence in ZnGa2O4 is reported in refs 13 and 14. The optical absorption spectra of a series of spinel samples Li[Ni0.5Mn1.5‑xTix]O4 is detailed in ref 15 for various concentrations (x). The luminescence characteristics of the ZnAl2O4:Mn nanophosphor are analyzed in ref 16, and the

1. INTRODUCTION Red emission of the Mn4+ ions incorporated in different materials is used in various applications, such as lighting,1−6 holographic recording,7,8 and thermoluminescence dosimetry.9 Tetravalent manganese is a promising candidate for replacing the well-established rare-earth Eu3+ activator in red phosphors, since the availability of rare earths is expected to be significantly limited in the near future. Mn4+ emission can improve the color-rendering index of phosphor-converted white-lightemitting diodes.10 For example, the quality of the YAG:Ce white light can be improved with the addition of a red-emitting phosphor that can be efficiently excited by blue light (∼470 nm).10 Additionally, deep-red emission complements the use of phosphors in biolabeling because biological species absorb less radiation in the higher-wavelength region of the visible spectrum. Here, the electronic configuration of the Mn4+ ions is 3d3; in a crystal field, the 8 LS terms of such an ion in a free state are split into a number of energy levels, which can be analyzed by the Tanabe−Sugano diagram.11 Mn4+ as an impurity can be incorporated into a number of materials, which can be conditionally referred to as “oxides” (if the surrounding environment is made of oxygen ions) and “fluorides” (if surrounded by fluorine ions). As a rule, the Mn4+ ions always encounter a so-called strong crystal field, which is characterized by a sharp red emission resulting from the spin-forbidden 2Eg © 2014 American Chemical Society

Received: September 22, 2014 Revised: November 15, 2014 Published: December 10, 2014 724

DOI: 10.1021/jp5095646 J. Phys. Chem. C 2015, 119, 724−730

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Figure 1. (a) HRTEM image and SAED pattern (inset) of Mg2TiO4 nanoparticles. (b) A subsection of the same region at higher magnification.

sol−gel preparation of the Mg2TiO4:Mn4+ phosphor is detailed in ref 17. With the emergence of nanoscience and nanotechnology, nanophosphorsnanoscale phosphorescent materialshave become the subject of extensive research 18 and have consequently exhibited novel properties,19 leading to many nanoscale applications, especially in various high-performance displays and devices. Although quantum size effects are much less pronounced with nanophosphors than with semiconducting quantum dots or noble-metal nanoparticles (NPs), the reduction of phosphor dimensionality provides many interesting features and functionalities. Specifically, nanophosphors have reduced (or negligible) light scattering compared to bulk powders (which is important for overall phosphor efficiency), their emission kinetics are altered (in most cases emission decays are longer compared to those in bulk), and they allow for the tuning of optical properties with nanocomposites (like in core−shell nanophosphors) or with crystal-field engineering (in solid solutions). Magnesium titanates are usually synthesized by a hightemperature solid-state reaction at ∼1400 °C, producing powders with relatively large, nonuniform particles.20−23 Reaction temperatures needed for Mg2TiO4 formation can be reduced with the use of wet chemical methods. To date, Mg2TiO4 particles have been successfully synthesized via sol− gel, coprecipitation, and polymeric precursor methods, as well as peroxide routes.10,24−27 So far, the smallest Mg2TiO4 particles (∼40 nm in diameter) were obtained using a peroxide route with a temperature treatment at 550 °C for 8 h.27 In the present work, we report on the synthesis of Mn4+ doped Mg2TiO4 NPs of about 10 nm in diameter. We then present a detailed analysis on their morphology and structure. We also provide results of spectroscopic and crystal-field studies of Mn4+ emission for these inverse-spinel Mg2TiO4 nanophosphors.

99.5%) was added to this solution and stirred until complete dissolution was observed. Next, a prescribed amount of MgO (Alfa Aesar, 96%) was dissolved in hot, concentrated nitric acid, dried by evaporation and added to a mixture of titanium(IV)isopropoxide/EG/CA and manganese nitrate (Aldrich, 99.9%). This mixture was stirred for 1 h at 60 °C until it became transparent and was stirred further at 130 °C for a few hours to promote polymerization and remove excess solvents, resulting in a resin-like product. In order to obtain black, amorphous precursor, the resin was fired at 350 °C for 30 min and ground to a powder. The precursor powder was further calcined at 600 °C for 1 h to produce pure-phase Mn4+ doped Mg2TiO4 nanoparticles. 2.2. Instruments and Measurements. The size, morphology, and crystallinity of nanoparticles were studied by high-resolution transmission electron microscopy (HRTEM), a 200 kV Cs-probe-corrected cold-field-emission transmission electron microscope (Jeol ARM 200 CF), coupled with a Gatan Quantum ER electron energy-loss spectroscopy (EELS) system and an energy-dispersive X-ray spectroscope (Jeol Centurio 100 mm2). Samples were dispersed in ethanol and placed on a copper lacy-carbon grid. EELS and EDXS spectra and mapping were collected in scanning-transmission mode (STEM). X-ray diffraction measurements were performed using a Rigaku SmartLab diffractometer. Diffraction data were recorded in the 2θ range from 10° to 90°, at a counting speed of 5°/min in 0.02° steps. Parameters for the samples’ crystal structure were derived from a Rietveld refinement of the XRD data using Topas Academic software. Photoluminescence measurements were performed at room temperature with a Fluorolog-3 Model FL3−221 spectrofluorometer system (Horiba JobinYvon), utilizing a 450 W xenon lamp as the excitation source for emission measurements and a xenon−mercury pulsed lamp for lifetime measurements. The emission spectra were scanned over the wavelength range of 425 to 740 nm under 385 nm excitation. The excitation spectrum was recorded over the wavelength range of 325 to 600 nm by monitoring the emission at 660 nm. The TBX-04-D PMT (Horiba JobinYvon) detector was used for both lifetime and steady-state acquisitions. The line intensities and positions of the measured spectra were calibrated with a standard mercury−argon lamp. Photoluminescence measurements were performed on pellets prepared from the NP powders under a load of 5 tons and

2. MATERIALS AND METHODS 2.1. Synthesis of Mn4+ Doped Mg2TiO4 Nanoparticles. For the synthesis of 2 wt % Mn 4+ doped Mg 2 TiO 4 nanoparticles, titanium(IV)-isopropoxide, magnesium(II)-nitrate, citric acid, and ethylene glycol were mixed in a 1:2:5:20 molar ratio. In the first step, titanium(IV)-isopropoxide (Alfa Aesar, 97%) was dissolved in ethylene glycol (Lach-Ner, 99%) under constant magnetic stirring. Then, citric acid (Kemika, 725

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Figure 2. (a) EELS spectrum of a Mn-doped Mg2TiO4 sample and (b) background-subtracted Mn L lines.

without any additives. Diffuse spectral reflectance measurements were performed on the Thermo Evolution 600 spectrometer equipped with an integrating sphere, using BaSO4 as a blank. Crystal-field calculations were performed using the exchange-charge model (ECM),28 which permits the analysis of crystal-field effects without requiring any a priori assumptions regarding the point symmetry of the impurity ion’s site.

3. RESULTS AND DISCUSSION 3.1. Size and Morphology of Mn4+ Doped Mg2TiO4 NPs. The structure and morphology of Mn4+ doped Mg2TiO4 powder were analyzed with TEM/STEM, EELS, and EDXS. It was found that the powder consisted of NPs of about 10 nm in diameter, agglomerated into micron-size particles. A lowermagnification figure (Figure 1a) reveals a mesoporosity of a few nanometers. Uniform, relatively sharp rings with visible individual spots in the selected-area electron diffraction (SAED) pattern (inset) corresponded to a Mg2TiO4 (Fd3mS) spinel structure and was characteristic of randomly oriented, roughly 10 nm particles. No other crystalline phase was detected with SAED. Examining the terminal planes of the nanoparticles, we found that no substantial amorphous phase was present near the crystals. EDXS analysis showed a homogeneous distribution of cations in NPs with about 2 wt % of Mn. EELS spectra reveal small Mn L3 and L2 edges at 642 and 652 eV with an approximate L3/L2 ratio of 1.2. On the basis of studies29 reporting differences in EELS spectra as a function of the valence state of manganese, such as when L3 and L2 lines are about 10 eV apart and the ratio of the lines is less than 2, we can assume that the Mn is in the 4+ valence state. In Figure 2a, an EELS spectrum with Ti, O, and Mn edges is presented. In Figure 2b the background-subtracted L3 and L2 lines of Mn are shown. Using the Gatan EELS Advisor code for simulating EELS spectra,30 we found that 2 wt % of Mn is just above the detection limit, meaning that the valence state of manganese could not be measured with the high precision. 3.2. Crystal Structure of NPs. An X-ray diffraction pattern for Mn4+ doped Mg2TiO4 NPs is shown in Figure 3 as a black line. The main reflections were indexed according to an ICDD

Figure 3. Rietveld data fit (red line) for powder XRD data (black line) from the Mn4+ doped Mg2TiO4 NPs and the difference pattern between simulated and experimental data (blue line); main reflections are indexed according to an ICDD 01-072-6968 card.

01-072-6968 card, and no reflections from impurity phases were detected. The Rietveld fit of the powder XRD data is also shown in Figure 3 as a red line. This fit produced a value of 8.43 Å for the lattice constant and 6.3 nm for the average crystallite size of the powder. Mg2TiO4 is a classic example of an inverse spinel,31−33 in which the tetrahedral sites are solely occupied by Mg2+ ions, and the octahedral sites are occupied by both Mg2+ and Ti4+ ions in a 1:1 ratio. It has been shown33 that with increasing temperature the structure becomes disordered in the sense that a small fraction of the Ti4+ ions can also be found in the tetrahedral sites. The refined value of the lattice constant of 8.43 Å has a similar value with those earlier reported: 8.41 Å31 and 8.44−8.45 Å.32 Figure 4 shows one unit cell of Mg2TiO4. It should be mentioned that the octahedral sites are slightly deformed in Figure 5. Although deviation of the octahedral environment around the Mg2+/Ti4+ ions from the ideal octahedral symmetry is considerable (none of the OMg2+/Ti4+O angles is equal to 90°), the oxygen octahedrons around each Mg2+/Ti4+ ion possess the trigonal symmetry C3i,35 and the third-order axis 726

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Figure 4. One unit cell of Mg2TiO4 drawn with VESTA.34 The Mg2+ ions at the tetrahedral positions are shown as separate spheres; the octahedral coordination of the Mg2+/Ti4+ ions at the 6-fold coordinated sites is also shown.

crystal-field parameters (CFPs), which can be calculated from the crystal-structure data without making any a priori assumptions regarding the symmetry of the impurity ion’s site. The CFPs reflect the arrangement of the host lattice ions around the impurity site and, as such, contain all structural information pertinent to calculating the CF splittings. The Hamiltonian (eq 1) is defined in the space spanned by all wave functions of the free ion’s LS terms (which arise due to the Coulomb interaction between the electrons of an impurity ion). The ECM allows us to represent the CFPs as a sum of two contributions:28 Bpk = Bpk, q + Bpk, S

(2)

with, Bpk, q = −K pke 2⟨r p⟩ ∑ qi

Figure 5. Mg/Ti octahedral coordination for Mg2TiO4 showing the interatomic distances and angles between the chemical bonds.

V pk(θi , ϕi) R i p+1

i

(3)

and coincides with the (−111) direction in the crystal lattice. Therefore, the Mn4+ ions, which occupy these octahedral sites, should experience the trigonal crystal field, which should partially raise the degeneracy of the Mn4+ electronic states. 3.3. Details for the Crystal-Field Calculations. Analysis of the symmetry properties of the Mn4+ positioning facilitates the application of the crystal-field (CF) theory, as will be shown in the following discussion. The energy levels of the Mn4+ ions in the present work were calculated as eigenvalues of the CF Hamiltonian:28

Bpk, S = K pke 2

+ γpGπ S(π )i 2 )

∑ ∑ p = 2,4 k =−p

∑ (GsS(s)i 2 + Gσ S(σ )i 2 i

V pk(θi , ϕi) Ri

(4)

The first term is the point-charge contribution to the CFPs, which is due to the Coulomb interaction between the impurity ion and the lattice ions, enumerated by index i with charges qi and spherical coordinates Ri,θi,φi (with the reference system centered on the impurity ion). At this point, the impurity ion and ligands are treated as point charges. The averaged values ⟨rp⟩, where r is the radial coordinate of the d electrons of the optical center (also known as the moments of the 3d electron density), can be easily calculated numerically, using the radial parts of the corresponding ion’s wave functions. The numerical factors K pk ,γ p , the expressions for the Bkp,q

p

H=

2(2p + 1) 5

Bpk Opk (1)

Okp

where are suitable linear combinations of the irreducible tensor operators acting on the angular parts of the impurity ion’s wave functions (exact definitions of the operators used in the ECM are given in ref 28), whereas the Bkp entries are the 727

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Table 1. Calculated Values of CFPs (Stevens Normalization, cm−1; the ECM Parameter G = 8.2) for Mn4+ in Mg2TiO4 in Comparison with Other Trigonal Centers Formed by Mn4+ Mg2TiO4: Mn4+ (this work) CFP

Bkp,q

Bkp,s

B02 B04 B34

685.8 −419.0 −11158.9

2970.3 −3662.4 −85623.4

Y2Ti2O7 total value

Bkp,q

+

44

Bkp,s

total value −5483.8 −2769.2 104715.9

3656.1 −4081.4 −96782.3

Y2Sn2O7

44

Bkp,q

Na2SiF6 +

45

Bkp,s

−7395.1 −2434.4 108135.5

−503.5 −3758.1 −108161.2

Table 2. Calculated and Experimental Energy Levels (in cm−1) for Mn4+ in Mg2TiO4 C3i group notation

calcd. in cm−1

Ag {2Eg(1) + 2Eg(2)} {2Eg(1) + 2Eg(2)} + 2Ag {4Eg(1) + 4Eg(2)} + 4Ag 2 Ag+{2Eg(1) + 2Eg(2)} 4 Ag+{4Eg(1) + 4Eg(2)}

0 15194a 15927a, 16547 20454a, 21307 22288, 23985a 27651, 29304a

{4Eg(1) + 4Eg(2)} + 4Ag

44112a, 48120

Oh group notation and “parent” LS term 4

4

A2g ( F) Eg (2G) 2 T1g (2G) 4 T2g (4F) 2 T2g (2G) 4 T1g (4F) ... 4 T1g (4P) 2

a

4

exp. in cm−1 15193 ∼20890 ∼28900 4

T1g (4P)

Orbital doublet states.

polynomials Vkp, and the definitions for the operators Okp are all given in refs 28 and 36 and thus are not shown here for the sake of brevity. The second term of eq 2, Bkp,S, is proportional to the overlap between the wave functions of the impurity ion and ligands to account for the covalent and exchange effects. At this point, the impurity ion and ligands are treated quantum mechanically, clearly distinguishing between different orbitals of the ions involved in chemical bond formation. The S(s), S(σ), S(π) terms denote the overlap integrals between the dfunctions of the impurity ion and the p- and s-functions of the ligands: S(s) = ⟨d0|s0⟩, S(σ) = ⟨d0|p0⟩, S(π) = ⟨d1|p1⟩. The Gs, Gσ, and Gπ coefficients represent the dimensionless adjustable parameters of the ECM, whose values are determined from the positions of the three lowest energy-absorption bands in the experimental spectrum. These can be taken as equal, (i.e., Gs = Gσ = Gπ = G), but then must be estimated by the lowest-energy absorption band. The summation in eq 4 includes only the nearest neighbors of an impurity ion (i.e., six ligands in the case of an octahedral impurity center), since the overlap with the ions from the second, third, etc. coordination spheres can be safely neglected. The ECM uses a small number of fitting parameters, which is one of the strongest points of the model. It also allows for calculating the CFPs and energy levels of impurities in crystals without invoking any assumptions about the impurity center symmetry. This is also very important for a consistent analysis of the low-symmetry crystal-field effects and comparative studies of isostructural/isoelectronic systems. The ECM has been successfully used for the calculations of energy levels of rare-earth ions28,37,38 and transition metal ions.37,39−41 3.4. Results of Crystal-Field Calculations and Emission Properties of NPs. Using the structural data from ref 31 and the refined lattice constant of 8.43 Å, we built up a cluster consisting of 56,630 ions, which allowed us to account for crystal-lattice ions located up to 75 Å from the impurity ion’s site. The Mn4+O2− overlap integrals were calculated numerically using the radial wave functions from refs 42 and 43. Applications of eqs 1−4 resulted in the following values for the CFPs (Table 1).

As seen in Table 1, the second contribution Bkp,s to the CFPs values is of paramount importance, being nearly eight times greater than Bkp,q in the case of B34. For a comparison, we also list in Table 1 the values of CFPs calculated for Mn4+ ions in other crystals with trigonal point symmetry. All the data in Table 1 are consistent: the absolute values of the corresponding CFPs are similar, and the difference in signs reflects opposite trigonal distortions (compression/elongation) along the third-order axis of rotation in different crystals. For the next step, the CF Hamiltonian (eq 1) with the CFPs from Table 1 was diagonalized in the space spanned by all wave functions of 8 LS terms for the d3 electron configuration of the Mn4+ ions. The Racah parameters B and C were taken as 790 and 3172 cm−1, respectively, which lie in the typical range for tetravalent manganese ions.36 The calculated energy levels of Mn4+ in Mg2TiO4 are collected in Table 2, in comparison with the experimentally deduced positions of the corresponding spectral peaks. There is a clear agreement here between the calculated and observed energy levels. The orbital triplets are split into singlet and doublet components, as should be the case for a trigonal crystal field. The irreducible representations of the C3i point group contain a one-dimensional representation Ag and two onedimensional complex-conjugate representations Eg(1) and Eg(2), which physically correspond to the doubly degenerate energy level. To emphasize this particular feature of the test cluster, two irreducible representations of Eg(1) and Eg(2) are shown in braces, according to the order of the calculated singlets and doublets. Figure 6 illustrates the relation between the calculated energy levels of Mn4+ ions in Mg2TiO4 and their experimental excitation and emission spectra. It can be seen that the main features of the excitation spectrum, corresponding to transitions from the 4A2g ground state to the 4T2g and 4T1g states (here, for the sake of brevity, the Oh-group notation is used) correlate well with the trigonal splitting of the orbital triplets. The emission-decay measurement at 658 nm (not presented here) exhibited single-exponential behavior with an excited-state lifetime of 1.2 ms. 728

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incorporates Mn4+ luminescence centers is obtained at a temperature of 600 °C, following 1 h of calcination. This is a much lower temperature and shorter time than those of traditional solid-state reactions, which require long firing cycles at temperatures of approximately 1400 °C. No signatures of manganese ions in divalent or trivalent valence states are observed in photoluminescence and diffuse reflection spectra. The energy levels of the Mn4+ ions in Mg2TiO4 were calculated using the crystal-field ECM. The point symmetry of the Mn4+ position is C3i, which is in agreement with the structure of the crystal-field Hamiltonian and calculated pattern of the energy-level splitting. The calculated energy levels of Mn4+ in a trigonal crystal field are in good agreement with the experimental excitation and emission spectra and allow for the designation of all prominent spectral features. The main excitation and emission peaks for these nanoparticles are very similar to the bulk material. The values of the CFPs for Mn4+ ions in Mg2TiO4 are similar to those calculated for Mn4+ in other crystals with trigonal point symmetry at Mn4+ sites. The crystal-field analysis also showed that contributions of covalent and exchange effects to CFPs are nearly eight times greater than the point-charge contribution.

Figure 6. Comparison of the calculated energy levels of Mn4+ in inverse-spinel Mg2TiO4 with experimental excitation (black line) and emission (red line) spectra.

As one can see from comparison of the refined lattice constant of NPs with the bulk value, the lattice constant for the NPs is slightly increased. As a result, a slightly lower value of the 10Dq parameter (crystal field strength) is observed in NPs in comparison to the bulk particles,35,46 but the main excitation and emission peaks for these nanoparticles are very similar to the bulk material. Figure 7 shows Kubelka−Munk function of the measured diffuse reflectance spectrum where characteristic absorption of



AUTHOR INFORMATION

Corresponding Author

*Tel. +381 11 3408607; e-mail: [email protected] Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS M.M., Ž .A., V.L., and M.D.D. acknowledge the financial support of the Ministry of Education and Science of the Republic of Serbia (Project No. 45020) and the support from the APV Provincial Secretariat for Science and Technological Development of the Republic of Serbia through Project No. 114-451-1850/2014-03. M.B. acknowledges the financial support of the Marie Curie Initial Training Network LUMINET through Grant Agreement 316906 and the Programme for the Foreign Experts offered by Chongqing University of Posts and Telecommunications. Goran Dražić acknowledges the financial support of the Slovenian Research Agency (ARRS) through Program No. P2-0148 and Project J26754.

Figure 7. Kubelka−Munk function, F(R), calculated from the diffuse reflectance spectrum of Mn4+:Mg2TiO4 NPs.



Mn4+ ions can be observed, and no Mn3+ spin-allowed absorption bands are present.

REFERENCES

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4. CONCLUSIONS Detailed spectroscopic and crystal-field studies of the Mg2TiO4:Mn4+ NPs are described in the present paper. Deep-red emitting (658 nm with a lifetime of 1.2 ms) Mg2TiO4:Mn4+ NPs of approximately 10 nm in diameter are obtained through the Pechini-type polymerized complex route, based on polyesterification between citric acid (CA) and ethylene glycol (EG) and the use of a mixed-metal CA complex with a stoichiometric Mg/Ti ratio of 2:1. Through this synthesis approach, an inverse-spinel crystal structure that 729

DOI: 10.1021/jp5095646 J. Phys. Chem. C 2015, 119, 724−730

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DOI: 10.1021/jp5095646 J. Phys. Chem. C 2015, 119, 724−730